Now, display cells used in flat panel displays, flat panel detectors, and the like are commonly produced by cutting out a plurality of display cells at once from a motherboard that is a large substrate. For example, liquid crystal cells which are typical display cells used in flat panel displays are produced as follows. A motherboard (hereinafter, also referred to as first motherboard) on which a plurality of panel substrates to be used as device substrates are arranged and a motherboard (hereinafter, also referred to as second motherboard) on which a plurality of panel substrates to be used as color filter (CF) substrates are arranged are bonded to each other by using a sealing material in a manner such that panel substrates on respective motherboards face to one another. Each panel substrate is cut out from the bonded motherboard to produce a plurality of cells. A liquid crystal is injected into the obtained cells by a method such as vacuum injection.
In the liquid crystal cell obtained by the above method, a device substrate is often equipped with a monolithic circuit including a thin film transistor that is made of a polycrystalline silicon thin film, in response to a recent demand for miniaturization of the frame region of a liquid crystal cell. Such a device substrate is formed as follows, for example.
First, an amorphous silicon thin film is formed on the principal surface of the first motherboard. Subsequently, this silicon thin film is polycrystallized by laser-annealing. And the obtained polycrystalline silicon thin film is patterned into a desired shape, and other required materials are also processed. Thereby, a plurality of panel substrates each equipped with a monolithic circuit including a thin film transistor are formed on the principal surface of the first motherboard.
In the process of laser-annealing, an excimer laser which can linearly irradiate a predetermined range is used, for example. In this process, it is ideal that the range irradiated with a laser light at once (the size of a laser light from one end to the other end, hereinafter, referred to as laser length) is larger than the width of the first motherboard. However, the excimer laser has the upper limit in the laser length because of the mechanism of the device and cost limitations. At present, the maximum laser length commonly used in a comparatively inexpensive device is about 300 mm.
On the other hand, the size of the first motherboard tends to increase from year to year. For example, a glass substrate having a size of about 730 mm×920 mm is already commercially available as the first motherboard. Enlargement of the excimer laser device is behind the enlargement of the first motherboard, and the width of the first motherboard is sometimes larger than the laser length.
Here, in the case of a first motherboard on which panel substrates are arranged in a matrix pattern, for example, excimer laser and the first motherboard are relatively moved along a direction of a line or a column in accordance with the arrangement of the panel substrates, and the excimer laser scans a predetermined part or the entire principal surface of the first motherboard for a plurality of times. Thereby, only the desired part or the entire surface of a silicon thin film is polycrystallized.
However, the procedure for forming a large number of panel substrates on the first motherboard is problematically complicated and time consuming in the method (for example, see Patent Document 1) of carrying out laser-annealing only on the required part of the silicon thin film.
Moreover, the method of carrying out laser-annealing on the entire surface of a motherboard is problematically time consuming because laser irradiation is performed also on the region which does not need to be polycrystallized.
Furthermore, the principal surface of a motherboard is scanned by a laser for a plurality of times in both methods. Therefore, when assuming that the region first irradiated by the excimer laser from one end to the other end of the motherboard is a first laser irradiation region and the region where the following laser irradiation is performed is a second laser irradiation region, there is a region called a laser joint region between the first laser irradiation region and the second laser irradiation region. The laser joint region is a region irradiated duplicately with a laser or not at all irradiated with a laser to keep an amorphous silicon thin film as it is.
The crystallization of the silicon thin film in the laser joint region is hardly controlled as it is intended to be. In addition, the crystallization degrees of the laser joint region, such as the average crystal grain size, the crystal grain size distribution, and the surface roughness of crystals, are significantly different from those of the first and second laser irradiation regions. Accordingly, a thin film transistor having high mobility is not appropriately formed in the laser joint region. Thus, since the laser joint region is not suitable for formation of a panel substrate, the number of the panel substrates which can be arranged on a motherboard is limited when the width of the laser joint region is large. This results in a larger useless region (hereinafter, referred to as wasted substrate region) including the laser joint region.
Moreover, as mentioned above, since the excimer laser has the upper limit in the laser length, the laser length may be slightly short depending on the size of a panel substrate. This may cause a case where the number of lines (columns) of the panel substrates which can be annealed by one laser irradiation on a motherboard is decreased. In the above example, the number of the panel substrates in each of the first and second laser irradiation regions is limited. Therefore, the number of the panel substrates which can be arranged on the whole motherboard is reduced, resulting in a problem of the additional production cost.
To solve the above problem, methods for irradiating a broader range with a laser light are disclosed, in which light from the light source is dispersed with use of a mirror and the like (for example, see Patent Documents 2 and 3). However, these methods require improvement of a laser irradiation device. In addition, a wasted substrate region mentioned above is still present between the first and second irradiation regions of laser beams which have been dispersed. Accordingly, efficient use of the motherboard can be still improved by reducing the wasted substrate region.
[Patent Document 1]
    JP-A Sho-63-11989[Patent Document 2]    JP-A Hei-11-186163[Patent Document 3]    JP-A 2000-12460